1. Field of the Invention
The present invention relates to a convenient and inexpensive analytical apparatus for determining the concentration of an interesting component in a sample without interference from coexistent components by measuring the light absorption of the interesting component.
More particularly, the present invention relates to the convenient and inexpensive analytical apparatus which is realized by the procedures of:
preparing an interference filter that has means for scanning a transmission wavelength periodically;
detecting a change in the intensity of light whose wavelengths are scanned by the interference filter and which transmit through the sample; and
determining a concentration of the interesting component by periodically examining the detected electric signal.
2. Description of the Related Art
It is well-known to detect an interesting component in a sample or to determine its concentration by measuring the light absorption of the interesting component. The simplest known method is to measure intensity changes of a monochromatic light transmitting through the sample in which the interesting component absorbing the monochromatic light is contained. For instance, an ultraviolet absorption photometer for monitoring water quality is well-known. This photometer utilizes a phenomenon in which an organic waste in water absorbs light of 253.7 nm from a mercury lamp, and the photometer consists of a light source, a sample cell, a detector and an amplifier. By this method, which is referred to as a single beam method, there often exists a measuring error because an interfering value due to a change in the intensity of the light source is not able to be distinguished from a measured value due to the light absorption of the organic waste.
There exists a double beam method which removes this defect of the single beam method. In the double beam method, a beam from a light source splits into two beams in which one of the beams is for a sensing beam and the other beam is for a reference beam, and the measured value is obtained as the difference or ratio between the intensities of these two beams. The intensity change of the sensing beam is standardized by the intensity of the reference beam. By adopting the double beam method, a change in the intensity of the light source does not affect the measured results.
Double wavelength spectrometry is another method which does not affect the measured results due to a change in the intensity of the light source. In this method, two different monochromatic beams are alternately transmitted through or reflected on a sample, and the absorption intensity of an interesting component in the sample is obtained by measuring the difference or ratio between the intensities of the two monochromatic beams. A wavelength of the one monochromatic beam for reference is selected separately from an absorption wavelength of the interesting component, and a wavelength of the other monochromatic beam for sensing is equal to the absorption wavelength of the interesting component. The intensity change of the monochromatic beam for sensing is standardized by the intensity of the monochromatic beam for reference. Therefore, this method does not have a measuring error due to a change in the intensity of the light source.
As the present invention is aimed at developing an apparatus which does not have a measuring error due to a change in the intensity of the light source, like the double wavelength spectrometry method described above, a prior art method that is one kind of the double wavelength spectrometry will now be explained with reference to FIG. 1.
FIG. 1 is a block diagram of a filter correlation infrared analyzer in which an infrared beam 121 radiated from an infrared source 101 is transmitted through a sample cell 102. An infrared beam 122, which is transmitted from the sample cell 102, is transmitted to a modulator 103. The sample cell 102 is a pipe in which infrared transmission windows are disposed on both ends thereof and which includes an inlet and an outlet on the near places from the both ends. An interesting component in a sample gas flowing through the sample cell absorbs infrared of a specific wavelength, and the infrared beam 122 transmitted through the sample cell 102 loses the energy at the specific wavelength. The modulator 103 is a rotating disk with filters mounted thereon, where one of the filters is a sensing filter 104 whose maximum transmission wavelength is equal to the specific wavelength absorbed by the interesting component, and the other filter is a reference filter 105 whose maximum transmission wavelength is different from the specific wavelength. By rotating the modulator, the filters periodically cross the infrared beam 122 transmitted through the sample cell 102. The infrared beam 122 transmitted through the sample cell 120 is modulated by the beam being transmitted through the sensing filter 104 and the reference filter 105 alternately. The modulated infrared beam 123 is focused on an infrared sensor 108 by a focusing lens 107, and the modulated infrared beam 123 is detected as an electric signal. The detected electric signal is amplified by a head amplifier 110, and is then output to a synchronous rectifier 111. On the other hand, a synchronous signal detector 106 detects a synchronous signal of the modulator 103, and transmits the synchronous signal to a phase adjuster 109. The phase adjuster 109 adjusts the synchronous signal and outputs the signal in the synchronous rectifier 111. The synchronous rectifier 111 rectifies the electric signal synchronizing with the synchronous signal, and thereby obtains a measured signal corresponding to the concentration of the interesting component in the sample.
By more careful consideration, the gaseous or liquid samples contain many coexisting components with the interesting component. Some of the components interfere with analyzing the interesting component, such as water vapor in the case of analyzing nitrogen mono-oxide in flue gas and water or electrolytes in water in the case of analyzing glucose dissolved in water.
Details of the interference caused by the coexisting components will be described with reference to FIG. 2. A spectrum of interfering components 152 has no peak at an absorption wavelength of the interesting component 153 but tails over it. Even if the slope of the tailing is small, the tailing often causes fatal errors in the analysis results because of an increasing concentration of the interfering components. The double wavelength spectrometry often has a problem in that the tailing of spectrum of the interfering components causes fatal errors in the analysis results even if the reference wavelength 154 is selected near the absorption wavelength of the interesting component 153.
Accordingly, an objective of the present invention is to provide an analyzing apparatus which removes the interference caused by the tailing spectrum of the interfering components, and which determines an exact concentration of the interesting component by measuring the light absorption of the interesting component. It is a further object of the present invention to provide a convenient and inexpensive analyzing apparatus capable of measuring interesting components in a manner that is neither destructive nor invasive to a person.
The present invention provides an interference filter transmission wavelength scanning photometer for determining the concentration of an interesting component in a sample without interference from the coexistent components by measuring the light absorption of the interesting component.
The photometer of the present invention comprises a light source, an interference filter operable to scan a transmission wavelength periodically, an infrared sensor, and a zero cross detector.
The photometer of the present invention can determine the concentration of an interesting component in a sample without interference from coexisting components in the sample by an operation comprising the following steps:
deciding a range for scanning the transmission wavelength of the interference filter so that the center of a small change of a wavelength induced by periodically scanning the transmission wavelength is equal to an absorption wavelength of the interesting component;
detecting an intensity of a light beam from the light source as an electric signal by the infrared sensor in which the light beam is provided to, after the light beam is transmitted through the interference filter and is transmitted to or reflected from the sample placed to the front or rear of the interference filter;
obtaining a full period T (the time for every other zero cross point of the AC component) and a half period T1(the time between the rise and fall of a zero cross point of the AC component) which the zero cross detector detects from the AC component of the electric signal;
determining the concentration of the interesting component by calculating a ratio according to the expression (T-2T1)/T.
Incidentally, a number of methods for scanning the transmission wavelength of the interference filter are well-known. One method scans the transmission wavelength by changing the refractive index of a spacer layer of gas which pressurizes up and down, which operates to keep the displacement between metal layers constant. In another method, a displacement between metal layers is changed so as to scan the transmission wavelength. In yet another method, which is suitable for the present invention, the transmittance wavelength is scanned by inclining the normal of the interference filter to the optical axis. The best mode for carrying out the present invention is an interference filter transmission wavelength scanning photometer wherein the transmission wavelength is scanned by swinging the interference filter periodically, which is achieved by inclining the normal of the interference filter to the optical axis.